As a spontaneous light emission type display device which employs light emitting elements, there has been known one which employs LEDs or inorganic EL elements as the light emitting elements. Besides, in recent years, an organic EL display device which employs organic EL elements as the light emitting elements has been often employed for, for example, an information display panel for use in an audio equipment, or an instrument panel for an automobile. In order to display a complicated pattern, a matrix type display device in which pluralities of light emitting elements are arranged vertically and laterally is adopted. Such matrix type display devices include two sorts; a passive matrix type wherein data electrodes (column electrodes) and scanning electrodes (row electrodes) are formed in the shape of a simple lattice and wherein the elements are caused to emit lights by duty-driving these elements for only the selection periods of the scanning electrodes, and an active matrix type wherein switching transistors are disposed in one-to-one correspondence with the individual elements and wherein the corresponding elements are statically driven by the respective transistors. Since the display device of the passive matrix type is simple in structure and low in price, the products thereof have been incarnated for an image display (for example, still picture display) in the automotive instrument panel, and so forth.
The spontaneous light emission type display device as stated above is capable of direct light adjustment in accordance with the output intensity of each light emitting element itself, unlike a non-spontaneous light emission type display device such as liquid-crystal display device. However, the intensity fluctuates with the lapse of time on account of the degradation of the light emitting element, etc., and a countermeasure needs to be taken. By way of example, in the case of the organic EL display device, the degradation occurs between an organic layer constituting the organic EL element and the interface of a cathode electrode made of a metal, or the cathode electrode itself is degraded by corrosion or the like, resulting in the problem that the intensity fluctuation is liable to occur. Especially, since the passive matrix type display device is of the duty drive scheme, it needs to light up each element at a high intensity instantly, and the degradation of its element tends to proceed more than in the static drive type.
It has been attempted to cope with the degradation of the element having occurred, by the stabilization of a supply voltage, or the like. Merely by the stabilization of the supply voltage, however, it has hitherto been difficult to satisfactorily cope with the intensity fluctuation of each light emitting element. Besides, the on-vehicle display device has the problem that a battery voltage serving as the supply voltage is prone to fluctuate with a large width, depending upon a load situation or the situation of use of an alternator, or due to the degradation of a battery itself, or the like, so the voltage fluctuation is liable to exert direct influence on the drive voltage of the display device, in turn, the output intensity. In the case of, for example, the organic EL element, it has been known that the intensity changes exponentially in the voltage-intensity characteristic thereof, and that the characteristic changes greatly, depending upon temperatures. On the other hand, in the current density-intensity characteristic of the organic EL element, the intensity increases substantially in proportion to the current density. This is because the organic EL element includes in equivalent circuit-wise, a diode constituent which is a rectifying element, and an internal resistance constituent which appears in series with the diode constituent. The proportionality of the intensity to the current density is elucidated by a quantum-mechanical light emission mechanism in which current energy is converted into light energy on the basis of the light emission recombination process of carriers (electrons/holes). Besides, the nonlinear change of the current density (namely, the intensity) versus the voltage is elucidated by a non-ohmic characteristic peculiar to the diode. Further, the temperature dependency of the characteristic is elucidated by the resistance temperature dependency of the internal resistance constituent, the thermal excitation process of the carriers in the diode constituent, etc. Anyway, in such a spontaneous light emission type display device, it has been desired to dispose a light adjustment mechanism for the purpose of stabilizing the output intensities of the individual elements, or coping with an intensity alteration which conforms to a user's favorite.
In the passive matrix type display device, possible methods are broadly classified into two schemes; a scheme wherein the light emission intensity of the element is subjected to a voltage light-adjustment control, and a scheme wherein it is subjected to a current light-adjustment control. In the case of, for example, the organic EL display device, when the voltage drive control is intended, it is necessary to compensate the temperature characteristic of the intensity and to relieve the nonuniform temperature distribution of a panel. Besides, since the voltage-intensity characteristic of the element is abrupt, it is necessary to consider the setting of a subtle voltage value. On the other hand, when the current drive control is intended, the temperature compensation need not be considered. Moreover, since the current-intensity characteristic of the element is linear, the control of a current value, in turn, the intensity is easy. That is, current drive as disclosed in Patent Document 1 (JP-A-10-222127) is advantageous for the drive of the organic EL display device, and the current light-adjustment control scheme has been generally employed in case of adjusting light, as disclosed in Patent Document 2 (JP-A-2005-77656).
However, the constant-current control type driver IC of an organic EL display device commercially available has its current control range limited, and it is therefore incapable of adjusting light in a wide range. Especially, it has the difficulty of lacking in the stability of the light adjustment control of a low intensity side. Another problem is that, as a current value which is outputted becomes smaller, the intensity dispersion between output channels (sets of pixels connected in parallel by a data (column) electrode) enlarges more.
Thus, it is required for a passive matrix type display device to have a light adjustment function which can perform the light adjustment control of a low intensity side stably and inexpensively.